1. Field of the Invention
The present invention relates to an image-processing apparatus and an image-processing method that retrieves an image that is similar to an image that is inputted thereto.
2. Description of the Related Art
A variety of technologies for retrieving a similar image have been proposed. For example, there is a method of dividing an image into a plurality of blocks, and employing an image characteristic amount, i.e., a representative color, of each respective block, to perform a pattern match, thereby using a position information of the color to retrieve the similar image, e.g., Cited Patent Reference 1. The method disclosed according to the Cited Patent Reference 1, however, requires calculating the characteristic amount from the image overall at the time that the retrieval is conducted, and consequently, the retrieval becomes complicated in such a circumstance as when a specified object is cut out from within the image, or when a background color changes, for example.
A method has accordingly been proposed that uses a localized characteristic amount, i.e., a local characteristic amount, rather than the characteristic amount of the image overall, to retrieve the similar image. Such a method commences by extracting a characteristic point, i.e., a local characteristic point, from the image. The method continues by calculating a characteristic amount with regard to the local characteristic point, i.e., the local characteristic amount, from the local characteristic point and image information in a vicinity thereof. The retrieval of the image is performed by matching the respective local characteristic quantities.
A method has been proposed that is capable of retrieval with a high degree of precision with regard to such a technique as the foregoing that uses the local characteristic amount, even when an image is rotated, by defining the local characteristic amount as an amount that is configured of a plurality of elements that are invariant by rotation, e.g., Cited Non-patent Reference 1. In a similar fashion, there is also a method that uses the local characteristic amount to match a position of the image by defining the local characteristic amount as the amount that is configured of the plurality of elements that are invariant by rotation, e.g., Cited Patent Reference 2.
The Cited Patent Reference 1, the Cited Patent Reference 2, and the Cited
Non-patent Reference 1 are as follows:
                Cited Patent Reference 1: Japanese Patent Laid-Open No. 08-249349.        Cited Patent Reference 2: Japanese Patent Laid-Open No. 09-44665.        Cited Non-patent Reference 1: C. Schmid and R. Mohr, “Local grayvalue invariants for image retrieval,” IEEE Trans. PAMI., Vol.19, No.5, pp. 530-535, 1997.        
With regard to the method disclosed in either the Cited Patent Reference 2 or the Cited Non-patent Reference 1, however, in fact, a fluctuation arises in the local characteristic amount that is defined as being invariant by rotation according to the present method, the fluctuation being caused by a noise that emerges when the image is rotated. FIG. 10 depicts an example of the fluctuation in the local characteristic amount that is defined as being invariant by rotation. FIG. 10 depicts the fluctuation in the local characteristic amount that is defined in the Cited Non-patent Reference 1 as a scatter diagram. The diagram plots seven elements of the local characteristic amount that are calculated at a common point when an image is rotated from 0 degrees through to 90 degrees, at 15 degree intervals. A problem that results from the fluctuation in the local characteristic amount that is apparent in FIG. 10 is deterioration in the precision of the image retrieval.